Cross-over network and bias voltage supply for dynamic-electrostatic speaker system

ABSTRACT

Circuit for use with a speaker system comprising a dynamic speaker and an electrostatic speaker and including a network providing a cross-over point at a frequency of the order of about 500 to 1000 Hz., and further including a voltage multiplier rectifier network delivering bias voltage to the electrostatic speaker, the input for this network being arranged for connection either with a standard 117v 60 Hz. power supply line, or with the secondary of a transformer of which the primary is connected with the source of signal being reproduced by the speaker system.

United States Patent 11 1 ,i1m m 1 11 3,895,193 Bobb July 15, 1975 [5CROSS-OVER NETWORK AND BIAS 3,345,469 /1967 Rod 179/111 R VOLTAGE SUPPLYFOR 3,358,084 l2/l967 Ashworth l79/l 11 R 3,562,429 2/1971 West 179/111R DYNAMIC'ELECTROSTATIC SPEAKER 3,632,903 1/1972 Lange 179/111 R SYSTEMInventor: Lloyd J. Bobb, Glenside, Pa.

Chester C. Pond, Doylestown, Pa. a part interest Filed: Sept. 13, 1971Appl. No.: 180,131

Related US. Application Data Continuation of Ser. No. 826,972, May 22,1969, and a continuation-in-part of Ser. No. 820,888, May 1, 1969, Pat.No. 3,654,403, and Ser. No. 104,741, Jan. 7, 1971, which is acontinuation of Ser. No. 724,805, April 29, I968, abandoned.

Assignee:

US. Cl. 179/111 R; 179/1 D Int. Cl H04r 19/02 Field of Search 179/111 R,1 D, 1 AT References Cited UNITED STATES PATENTS 7/1959 Janszen l79/lllR 90 7/l96l Hupert et al l79/l D OTHER PUBLlCATIONS The Radio AmateursHandbook, th Edition, 1963, p. 239 & 240.

Primary Examinerl(athleen H. Claffy Assistant Examiner-Douglas W. OlmsAttorney, Agent, or Firm-Synnestvedt & Lechner [57] ABSTRACT Circuit foruse with a speaker system comprising a dynamic speaker and anelectrostatic speaker and including a network providing a cross-overpoint at a frequency of the order of about 500 to 1000 Hz., and furtherincluding a voltage multiplier rectifier network delivering bias voltageto the electrostatic speaker, the input for this network being arrangedfor connection either with a standard 117v Hz. power supply line, orwith the secondary of a transformer of which the primary is connectedwith the source of signal being reproduced by the speaker system.

5 Claims, 4 Drawing Figures PNENTEBJUL 15 ms n K E w 5 km w M 1% m 6 W m1 I U m CROSS-OVER NETWORK AND BIAS VOLTAGE SUPPLY FORDYNAMIC-ELECTROSTATIC SPEAKER SYSTEM This is a continuation, ofapplication Ser. No. 826,972, filed May 22, 1969. The presentapplication is also a continuation-impart of my prior application Ser.No. 820,888, filed May 1, l969,and issued Apr. 4, 1972, as US. Pat. No.3,654,403; and also of application Ser. No. 104,741 filed Jan. 7, 1971,as a continuation of application 724,805, filed Apr. 29, 1968, nowabandoned.

This invention relates to audio speaker systems and is particularlyconcerned with speaker systems incorporating a plurality of speakersadapted to cover different portions of the audio spectrum. In a typicalinstallation the low range speaker comprises a dynamic speaker and thehigh range speaker comprises an electrostatic speaker. The dynamicspeaker is preferably of the well known cone type, having a voice coil,and the electrostatic speaker is preferably of the type disclosed in mycopending application Ser. No. 820,888, filed May 1, 1969, and issuedApr. 4, 1972, as US. Pat. No. 3,654,403, most advantageously comprisinga pair of acoustically coupled push-pull driven planar electrostaticspeakers.

One of the major objectives of the invention is to pro vide a cross-overnetwork for use with speaker systems of the kind referred to, in whichthe response of the dynamic speaker is effectively limited to afrequency rang below a cross-over point which is relatively low ascompared with various prior dual speaker arrangements, for instance at afrequency of the order of 500 Hz. This is of advantage in providinghighest fidelity of response of the speaker system as a whole for resonswhich will be explained hereinafter.

Another objective of the invention is to provide a simplified biasvoltage supply system for the electrostatic speaker which may beconnected either with a standard I 17v 60 Hz. supply line oralternatively with the secondary of a transformer having its primaryintroduced in the signal circuit. In the latter alternative it is afurther objective and advantage of the invention that a separate powersource connection for the bias voltage need not be provided for theelectrostatic speaker, as has been customary heretofore.

It is also an objective of the invention to provide a voltage multiplierrectifier circuit including current isolating resistors in the inputconnection, so that, notwithstanding the fact that a biasing voltage ofthe order of 1000 volts or more is delivered to the electrostaticspeaker, the current is restricted to such a low level as to eliminateany danger of harmful electrical shock to any one coming in contact withthe equipment.

A further objective of the invention is to provide a cross-over networkwhich is exceedingly simple and inexpensive, and yet highly effective,and which at the same time retains a exceptionally high level offidelity in the frequency response of both the low and high rangespeakers.

How the foregoing and other objects and advantages are attained willappear more fully from the accompanying drawings, in which:

FIG. 1 is a schematic view of one embodiment of the cross-over networkand bias supply system of the present invention as used in associationwith a dynamic speaker and with a pair of acoustically coupledelectrostatic speakers arranged in accordance with the disclosure of mycopending application above identified;

FIG. 2 is an outline somewhat diagrammatic view illustrating certainfeatures of a signal transformer preferably employed to couple thesignal circuit with the electrostatic speaker;

FIG. 3 is a view illustrating certain portions of the equipment shown inFIG. 1 and embodying an alternative arrangement for the bias voltagesupply system for the electrostatic speaker; and

FIG. 4 is a view illustrating certain portions of a circuit similar tothat shown in FIG. 1 but applied to an alternative arrangement in whichonly a single electrostatic speaker is used.

Referring first to FIG. 1, the dynamic speaker is illustrateddiagrammatically as incorporating a cone 5 and a voice coil 6. Theelectrostatic speaker here shown diagrammatically includes a pair ofconductive membranes 77, formed, for example, of a film of resinmaterial carrying a conductive coating at a side thereof. As here shownit is contemplated that the conductive coatings of the two membranesface each other and that the membranes be in closely spaced relation soas to be acoustically coupled. The conductive coatings of the membranesmay be connected in common to the conductor 8. The electrostatic speakerarrangement shown in FIG. 1 also includes a series of backing elementsor plates 9 for each of the two membranes, most of these plates 9 beinginterconnected in pairs and the pairs being connected through resistorsto the conductors 10 and 11 in the manner shown. Details of thismultiple backing plate arrangement, including the electricalinterconnections therefor need not be considered herein, but referencemay be made to the copending application above identified for suchdetails.

In FIG. 1 the connections 12 and 13 represent the signal source orsignal circuit and it will be observed that the voice coil 6 isconnected in series with the primary winding 14 of the signaltransformer, the coil 6 and winding 14 being connected across the signalcircuit. A capacitor 15 is connected across the voice coil 6 and, incombination with the primary 14 of the signal transformer, serves as thecrossover network.

The secondary 16 of the signal transformer has its ends connected withthe conductors 10 and 11 which are associated with the backing plates ofthe electrostatic speakers. The signal transformer also has a center tapconnection 17 associated with the voltage multiplier rectifier circuitwhich is enclosed within the dotdash line 18.

The voltage multiplier rectifier is of a type known per se, including,for example six diodes as indicated at 19 and six capacitors asindicated at 20. The input connections 21 and 22 of this network includecurrent isolating impedences which may comprise resistors, inductancesor capacitors, preferably resistors such as indicated at 23 and 24adapted to be connected with the power supply line, for instance thestandard 117v Hz. supply line.

It will be noted that the negative output connection 25 of this networkis coupled with the center tap connection 17 of the secondary 16 of thesignal transformer. The positive output connection 26 of this network iscoupled through a resistor 27 with the connector 8 leading to theconductive coatings on the membranes 7. If it is desired to connect morethan one speaker covering the same frequency range, this may be done bycoupling one or more additional speakers in parallel and connecting themthrough the resistor 27. In this case it may be desirable to change thevalue of the resistor 27, for instance from the 2 megohm value referredto herein to l megohm. Where additional electrostatic speakers coveringthe same frequency range are to be incorporated the backing platesthereof would be connected in parallel with those of the speakerillustrated and fed in parallel from the connectors and 11. In this casesome change in the signal transformer may be desirable.

Where it is desired to connect additional electrostatic speakerscovering a different frequency range, it may be desirable to do thisthrough separate resistors such as indicated at 27a.

With the circuit arrangements described above, the multiplier rectifiernetwork will deliver a bias voltage of the order of 1000 volts, therebyproviding an appropriate bias across the elements of the electrostaticspeakers. At the same time the signal voltage will be impressed upon thetwo electrostatic speakers in pushpull manner, and for reasons fullybrought out in my copending application'above identified, this push-pulldrive of a pair of acoustically coupled electrostatic speakers providesan exceptionally high fidelity of reproduction and also provides forhigh efficiency, so that the electrostatic speakers need not be made ofexcessive size.

More or less than six diodes and six capacitors may be used in themultiplier rectifier circuit, depending upon whether more or less thanabout 1,000 volts is desired.

The provision of the current isolating resistors 23 and 24, which may,for example, be of the order of 10k to 100K ohms each, serves to limitthe current in the multiplier network and in the connections to theelectrostatic speakers to such a low value that danger of harmfulelectrical shock is eliminated. In a typical network according to thepresent invention, the diodes 19 should have PIV and current ratingadequate to provide the desired bias voltage, in the case of 1,000volts, the diode identified as 1Nl695 being usable. The capacitors 20may be of the order of 0.01 to 0.1 mfd., and

of appropriate working voltage. The resistor 27 may 7 suitably be of 2megohms.

With a signal transformer in which the primary 14 comprises about 34turns of No. 19 wire and in which the secondary comprises about 2800turns of No. 37 wire, and with a dynamic speaker having an 8 ohm voicecoil (6) and still further with a capacitor 15 of about mfd., aneffective cross-over point is provided at a frequency of the order of500 Hz. With a doubleended, push-pull driven electrostatic speakersystem as illustrated, the electrostatic speaker is effective inreproducing frequencies from the upper limit of audibility down to avalue approximating the cross-over point of 500 Hz. Moreover, thedynamic speaker, which is limited in its effective reproduction to afrequency of the order of 500 Hz. in the system as described, is notsubject to the distortions commonly encountered with dynamic speakers asa result of attempts to reproduce sound running up to frequencies of atleast several thousand Hz. Tests have shown that a dynamic speaker, forinstance of the well known cone type, functions to very accuratelyreproduce sound within the frequency range in which the cone can operateas a piston, this range ordinarily being limited at its upper end toabout 500 to 1,000 Hz. depending on the cone design.

In order to avoid excessive magnetization of the core of the signaltransformer 14-16 at frequencies at or below the cut-off point of thetransformer, with consequent tendency to saturate the core and thuscause distortion or non-linearity in the reproduction by the speakers ofthe system, a special core arrangement is preferably employed for thesignal transformer, having a gap in the magnetic circuit. Referring toFIG. 2, it will be seen that the core elements of the signal transformerinclude both E laminations 28 and I laminations 29, these being shapescustomarily employed. However, instead of building up the core byalternately positioning the E and I laminations at opposite sides of thecore, so that they interleave, the core is here built up by stacking allof the E laminations upon each other in congruent positions, while the llaminations are similarly stacked directly upon each other. The windingsare of course arranged to surround the center leg of the E laminations.In the arrangement of the invention, a non-conductive spacer 30, forinstance paper is introduced between the stack of E laminations and thestack of I laminations in order to provide an air gap in the core.Preferably the paper or spacer thickness should provide a gap in theflux path of the order of 0.002 to 0.005 inch. This gap in the flux pathreduces the magnetization of the core at frequencies below the cut-offof the transformer and thereby avoids saturation of the core, withconsequent non-linearity in the primary winding 14 which is connected inseries with the voice coil 6. If such non-linearity or distortion waspresent in the primary 14 of the signal transformer, this would alsoadversely influence the linearity of the signal in the secondary 16 ofthe transformer and thus in the signal as applied to the electrostaticspeakers.

With the arrangement as shown, a high degree of linearity and fidelityis provided, while at the same time minimizing the components used andthe complexity of the cross-over network which, in effect, is reducedvirtually to a single capacitor 15, in combination with the voice coilof the speaker and the primary 14 of the signal transformer.

Although the simplified network, with the transformer having an air gap,achieves various of the objectives above referred to, it neverthelessstill supplies some voltage to the dynamic speaker at higherfrequencies, but at frequencies upwards appreciably above 500 I-Iz., forinstance of the order of about 2,000 Hz., this voltage is not high and,because of the inherent inertia of dynamic speakers designed for lowfrequency use, the sound generated by the dynamic speaker in consequenceis negligible.

The voltage multiplier rectifier circuit provides an exceptionallysimple means for attaining an appropriate polarizing or biasing voltagefor use in the electrostatic speakers. However, since such a voltagemultiplier is line connected, i.e., is directly coupled with the powersupply line, in order to avoid danger of electrical shock from theequipment, current isolating resistors 23 and 24 are introduced. Inaddition, the isolating resistors 23 and 24 also protect the voltagemultiplier rectifier from drawing excessive current in case of componentfailure, thereby eliminating fire hazard and damage to other components.In the system of FIG. 1, moreover, it is of importance that thesecondary 16 of the signal transformer is electrically isolated from theprimary 14 of that transformer thereby avoiding introduction of a highvoltage of the multiplier and also of the supply line voltage into thesignal circuit 12-13.

The resistor 27 serves the purpose of reducing shock hazard by limitingcurrent to the speaker from the voltage multiplier rectifier, and alsoprotects the speaker membrane from excessive overload conditions andthereby reduces damage to and prevents destruction of the membrane.

In the alternative embodiment illustrated in FIG. 3, provision is madefor deriving the biasing voltage for the electrostatic speakers from thesignal source 12-13, instead of from a l 17v A.C. supply line. This isaccomplished by provision of the circuit enclosed in the dotdash line 31shown in FIG. 3. This circuit is adapted to be alternatively coupledwith the current isolating resistors 23 and 24 which are provided in theinput to the voltage multiplier rectifier, and it will be understoodthat the remainder of the circuit thereof, not illustrated in FIG. 3,will be the same as in FIG. 1. In the circuit arrangement 31 a voltagestep-up transformer is employed, having a primary winding 32 and asecondary winding 33. The primary winding is connected into the signalcircuit 12-13, and thus is placed in series with the voice coil 6 andthe primary 14 of the signal transformer. The output connections of thesecondary winding 33 are coupled with resistors 34 which in turn arebridged by one or more gas discharge voltage regulating devices, forinstance, a pair of series connected 60v neon voltage limiters 35, thiscircuit 31 being adapted to be connected with the current blockingresistors 23 and 24 in the manner indicated in FIG. 3. Although otherkinds of voltage limiting devices may be used, the neon limiters are notonly effective but are also of very low cost.

If desired the primary 32 of the transformer 32-33 may be designed tooperate in parallel with the signal source.

Having in mind that the signal circuit 12-13 will carry a fluctuatingvoltage in accordance with fluctuations of the signal, and furtherhaving in mind that the signal voltage may range up to about 25 volts,in the embodiment illustrated, the transformer comprising the primaryand secondary windings 32 and 33 advantageously is wound to provide astep-up ratio of the order of 1:1000. With typical signal voltages, thevoltage across the secondary will at the minimum be of the order of 100volts, although will frequently be much higher than 100 volts. The neonvoltage limiters 35, for instance comprising two NE-Z, will provide avoltage limitation appearing at the terminals to be connected with theresistors 23 and 24, so that the voltage at those terminals will remainat a value of the order of 100 to 120 volts. This voltage will then bemultiplied by the multiplier rectifier circuit as described above inconnection with FIG. 1, in order to attain a bias voltage of the orderof I l ,000 volts, which is appropriate to deliver to the electrostaticspeakers.

While the system of FIG. 3 will use a portion of the output power of thepower amplifier feeding the signal circuit 12-13, the total amount ofpower required to establish the desired bias voltage for theelectrostatic speakers is of the order of 0.5 milliwatt, neglectingtransformer loses, and this drain upon the signal level isinsignificant, having in mind that it is common for power amplifiers todeliver upwards of 5 or watts.

In FIG. 4 an alternative connection system is illustrated for use in aninstallation where only a single electrostatic speaker is employed,instead of the multiple, push-pull driven arrangement of FIG. 1. Here itwill be seen that the electrostatic speaker comprises a single membrane7a and a single set of backing plates 9a, which are coupled to the lead10a in a manner similar to that illustrated in FIG. 1. Lead 10a isconnected to one end of the secondary 16 of the signal transformerthrough a capacitor 36. The connections from the voltage multiplierrectifier circuit are indicated at 25 and 26, as in FIG. 1, and thepositive connection 26 is extended to the head 37 which interconnectsone end of the secondary 16 of the signal transformer with theconductive coating on the membrane 7a, a resistor 27 being introduced inthe connection 26, as in FIG. 1.

The negative connection 25 of the voltage multiplier rectifier isextended to the lead 10a which in turn is connected with the backingplates 9a of the electrostatic speaker. The capacitor 36 should be ofthe order of about ten times the capacity of the speaker. Since thecapacity of the speaker in a typical construction would be of the orderof 0.01 mfd., the capacitor 36 should be of the order of 0.1 mfd. Thecapacitor 36 is introduced to prevent short circuiting of the DC.biasing voltage.

In the embodiment of FIG. 4, the resistor 27, in addition to thefunctions described above with relation to FIG. 1, further serves toprevent the bias supply from loading the secondary 16 of the signaltransformer.

In the arrangement of both FIGS. 1 and 4 it will be understood that theresistor 27, which may, be of the order of 1 to 10 megohms, may beintroduced in either output connection from the voltage multiplier. Itshould further be understood that the positive output connection of thevoltage multiplier need not necessarily be connected with the membranesof the electrostatic speaker or speakers, and the negative output withthe backing plates. The negative output connection may be extended tothe membranes and the positive connection to the backing plates. Othercircuit connections may be adopted, so long as provision is made for thebias voltage to be applied between the membranes and the backing platesof the electrostatic speakers.

In addition to providing a cross-over network of exceptional simplicity,the circuit arrangements of the present invention also achieve theproduction of an appropriate bias voltage for the electrostatic speakerwith parts of very low total cost, and both the cross-over network andthe bias voltage supply are coupled with the speakers without impairingthe operation of each other and further without danger, of harmfulelectrical shock notwithstanding the delivery of a bias voltage to theelectrostatic speaker as high as 1000 volts or more.

As above indicated the low frequency speaker preferably comprises adynamic speaker, but certain features of the invention are alsoapplicable to dual or even other multispeaker systems in whichelectrostatic speakers are even used for both the high and low ends ofthe frequency range. In such an arrangement, a primary of another signaltransformer may be substituted for the voice coil 6 shown in thedrawings, with the secondary of the other signal transformer connectedto feed another electrostatic speaker adapted for the low frequencyrange.

I claim:

1. For use in combination with a signal circuit, a dynamic speakerhaving a voice coil, an electrostatic speaker having a conductivemembrane and having a conductive backing plate cooperating with saidmembrane, and further with a standard 117 v. alternating current sourceadapted to be used in providing a bias voltage for the electrostaticspeaker: a step-up signal transformer having a primary winding connectedin the signal circuit in series with said voice coil, a solid statevoltage multiplier rectifier having a pair of input connections lineconnected with said current source each through a separate currentisolating resistor and having one output terminal connected with themembrane, the signal transformer having a secondary winding connectedbetween the backing plate of the electrostatic speaker and the otheroutput terminal of the voltage multiplier rectifier.

2. Equipment as defined in claim 1 in which said signal transformercomprises E and 1 core structures each comprising a plurality oflaminations, with the primary and secondary windings surrounding thecentral leg of the E laminations, at least some portions of the E and llaminations being spaced in fixed relation to each other to provide aconstant gap in the flux path.

3. For use in combination with a signal circuit, a low frequencyspeaker, an electrostatic high frequency speaker having a conductivemembrane and having a conductive backing plate cooperating with saidmembrane, and further with a standard 117 v. alternating current sourceadapted to be used in providing a bias voltage for the electrostaticspeaker: connections for delivering signal voltage to the low frequencyspeaker,

a step-up signal transformer having a primary winding connected in thesignal circuit, a solid state voltage multiplier rectifier having a pairof input connections line connected with said current source eachthrough a separate current isolating resistor and having one outputterminal connected with the membrane, the signal transformer having asecondary winding connected between the backing plate of theelectrostatic speaker and the other output terminal of the voltagemultiplier rectifier.

4. For use in combination with a signal circuit, an electrostaticspeaker having conductive membrane and cooperating backing plateelements, and further with a standard 117 v. alternating current sourceadapted to be used in providing a bias voltage for the speaker: a solidstate voltage multiplier rectifier providing said bias voltage andhaving output terminals connected to deliver the bias voltage to themembrane and backing plate elements, a step-up signal transformer havinga primary'winding connected in the signal circuit and having a secondarywinding connected with the membrane and backing plate elements butconductively isolated from the signal circuit, the voltage multiplierrectifier having input connections line connected with said alternatingcurrent source, and a separate current isolating resistor in each ofsaid input connections.

5. Equipment as defined in claim 4 and further including a currentlimiting resistor in the connection of the solid state voltagemultiplier rectifier'with at least one of the speaker elements.

1. For use in combination with a signal circuit, a dynamic speakerhaving a voice coil, an electrostatic speaker having a conductivemembrane and having a conductive backing plate cooperating with saidmembrane, and further with a standard 117 v. alternating current sourceadapted to be used in providing a bias voltage for the electrostaticspeaker: a step-up signal transformer having a primary winding connectedin the signal circuit in series with said voice coil, a solid statevoltage multiplier rectifier having a pair of input connections lineconnected with said current source each through a separate currentisolating resistor and having one output terminal connected with themembrane, the signal transformer having a secondary winding connectedbetween the backing plate of the electrostatic speaker and the otheroutput terminal of the voltage multiplier rectifier.
 2. Equipment asdefined in claim 1 in which said signal transformer comprises E and Icore structures each comprising a plurality of laminations, with theprimary and secondary windings surrounding the central leg of the Elaminations, at least some portions of the E and I laminations beingspaced in fixed relation to each other to provide a constant gap in theflux path.
 3. For use in combination with a signal circuit, a lowfrequency speaker, an electrostatic high frequency speaker having aconductive membrane and having a conductive backing plate cooperatingWith said membrane, and further with a standard 117 v. alternatingcurrent source adapted to be used in providing a bias voltage for theelectrostatic speaker: connections for delivering signal voltage to thelow frequency speaker, a step-up signal transformer having a primarywinding connected in the signal circuit, a solid state voltagemultiplier rectifier having a pair of input connections line connectedwith said current source each through a separate current isolatingresistor and having one output terminal connected with the membrane, thesignal transformer having a secondary winding connected between thebacking plate of the electrostatic speaker and the other output terminalof the voltage multiplier rectifier.
 4. For use in combination with asignal circuit, an electrostatic speaker having conductive membrane andcooperating backing plate elements, and further with a standard 117 v.alternating current source adapted to be used in providing a biasvoltage for the speaker: a solid state voltage multiplier rectifierproviding said bias voltage and having output terminals connected todeliver the bias voltage to the membrane and backing plate elements, astep-up signal transformer having a primary winding connected in thesignal circuit and having a secondary winding connected with themembrane and backing plate elements but conductively isolated from thesignal circuit, the voltage multiplier rectifier having inputconnections line connected with said alternating current source, and aseparate current isolating resistor in each of said input connections.5. Equipment as defined in claim 4 and further including a currentlimiting resistor in the connection of the solid state voltagemultiplier rectifier with at least one of the speaker elements.